OXYGEN ISOTOPES IN OLIVINE REVEAL SMALL-SCALE VARIATIONS IN THE MANTLE BENEATH THE POISON LAKE CHAIN, LASSEN REGION, CALIFORNIA

We present new laser fluorination δ¹⁸O data obtained from olivine crystals separated from 39 samples of contemporaneous (100ka ± 10ka) basalts erupted in the Poison Lake chain (PLC). The Poison Lake chain is a series of predominantly calc-alkaline basalts erupted in a small area of <30km² located in the Lassen Region of the southern Cascade arc. Previous studies of PLC trace element and radiogenic isotopic compositions suggest that there are several chemically distinct groups of mantle- derived basalts that reflect a complex sub-arc mantle with components that may include: unmodified mantle, subduction fluids, subducted sediments, and/or slab melt. Oxygen isotopes help us to distinguish among these inputs when used with other geochemical data. New δ¹⁸O_ol data confirms small-scale complexity in the mantle. Typical δ¹⁸O_ol values for mantle olivine are 5.1‰ +/- 0.2‰; however, values from PLC samples range from 5.4‰ to 6.1‰. These isotope compositions can be subdivided into a main group of primitive basalts with lower δ¹⁸O_ol values and a smaller group of more evolved basalts with higher δ¹⁸O_ol values. High field strength elements (HFSE), large ion lithophile elements (LILEs) and La/Lu have positive correlations with δ¹⁸O_ol. There is little correlation among radiogenic isotopic values and δ¹⁸O_ol, although evolved samples with high HFSE and elevated δ¹⁸O show some elevation in Os isotopes (¹⁸⁷Os/¹⁸⁸Os = 0.176-0.230). Elevated HFSE with respect to δ¹⁸O_ol may suggest fractional crystallization or crustal contamination, consistent with more evolved samples. Elevated LILE, and Os isotope ratios are interpreted to indicate crustal or sediment input in continental arc magmas. We combine δ¹⁸O_ol and other geochemical data to demonstrate complexity in sub-arc mantle in the southern Cascade Range on a small scale. The range in δ¹⁸O_ol values and trace element values among calc-alkaline basalt groups in the PLC illustrates that mantle-derived rocks in the southern Cascades reflect a variety of subduction inputs into the mantle and demonstrate small-scale heterogeneity in the sub-arc mantle.